NEW YORK (TheStreet) -- Five of 27 patients with an aggressive and hard-to-treat form of breast cancer responded to treatment with Merck's (MRK) checkpoint inhibitor Keytruda, according to interim results from an early-stage study announced Wednesday.
One of the five responding patients had a complete remission, which means there was no evidence of the cancer in her body. The other four patients had partial responses.
All the patients in the study had triple-negative breast cancer -- one of the most aggressive forms of the disease for which no targeted therapies have been successfully developed to date.
Merck said it will start a phase II study of Keytruda in triple-negative breast cancer in the first half of next year, supported by the clinical activity seen in the phase I study. Wednesday's Keytruda data were presented at the 2014 San Antonio Breast Cancer Symposium.
The class of immunotherapies known as checkpoint inhibitors work by blocking a protein known as PD-1 which cancer cells use to hide from a patient's immune system. Blocking PD-1 makes tumors visible to killer T cells.
Merck secured U.S. approval for Keytruda in advanced melanoma in September. Bristol-Myers Squibb's (BMY) Opdivo is approved for melanoma in Japan and is under review for the same indication in the U.S. Both companies are rushing ahead to develop their respective drugs in other solid tumors like lung cancer, and now breast cancer. Both drugs are also showing promising clinical activity in blood cancers, highlighted at presentations this past weekend.